A lead frame having a coating of organic compounds on its lead fingers prevents tin and flux from contaminating the lead fingers after die attach. The coating is removed prior to wire bonding. The coating allows for reliable second bonds (bond between wires and lead fingers) to be formed, decreasing the likelihood of non-stick and improving wire peel strength.
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1. A method of preparing a lead frame prior to attaching a semiconductor die to a die flag of the lead frame, the method comprising:
washing the lead frame in an acid bath;
performing a first water rinsing operation on the acid washed lead frame;
neutralizing the acid on the rinsed lead frame;
performing a second water rinsing operation on the lead frame;
disposing a coating solution on the lead fingers of the lead frame;
performing a third rinsing operation on the coated lead frame; and
drying the rinsed, coated lead frame.
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The present invention relates to the packaging of integrated circuits (ICs) and more particularly to coating the leads of a lead frame to prevent contamination of the leads during the die attach process.
In many types of semiconductor packages, especially those for high power devices, an integrated circuit die is attached to a die pad of a lead frame with solder paste and then pads on the die are electrically connected to lead fingers of the lead frame with wires. The solder paste is used in order to enhance reliability and thermal conductivity. However, when the die is attached to the die pad, some flux may be deposited on the lead fingers, such as during a reflow operation, causing the formation of tin clouds on the lead fingers. The flux and tin clouds can compromise the subsequent bond between the wire and the lead fingers, leading to bonds that don't stick or a bond with a low peel strength.
It would be advantageous to be able to attach a die to a lead frame with solder paste but not compromise the integrity of subsequent wire bonds to the lead fingers of the lead frame.
The following detailed description of preferred embodiments of the invention will be better understood when read in conjunction with the appended drawings. The present invention is illustrated by way of example and is not limited by the accompanying figures, in which like references indicate similar elements. It is to be understood that the drawings are not to scale and have been simplified for ease of understanding the invention.
The detailed description set forth below in connection with the appended drawings is intended as a description of a presently preferred embodiment of the invention, and is not intended to represent the only form in which the present invention may be practiced. It is to be understood that the same or equivalent functions may be accomplished by different embodiments that are intended to be encompassed within the spirit and scope of the invention. In the drawings, like numerals are used to indicate like elements throughout.
In one embodiment, the present invention provides a method of preparing a lead frame prior to attaching a semiconductor die to a die flag of the lead frame. The method includes the steps of washing the lead frame in an acid bath; performing a first water rinsing operation on the acid washed lead frame; neutralizing the acid on the rinsed lead frame; performing a second water rinsing operation on the lead frame; disposing a coating solution on the lead fingers of the lead frame; performing a third rinsing operation on the coated lead frame; and drying the rinsed, coated lead frame.
In another embodiment, the present invention provides a method of packaging a semiconductor integrated circuit, including the steps of providing a lead frame having a coating of organic compounds at least on lead fingers of the lead frame; attaching the semiconductor integrated circuit to a die flag of the lead frame using solder; performing a solder reflow operation on the lead frame and die attached thereto, wherein during said reflow operation, tin and flux contact the coated lead fingers; and removing the coating from the lead fingers, wherein removing said coating removes the tin and flux from the lead fingers.
A method of attaching a semiconductor integrated circuit or integrated circuit die to a lead frame will now be described with reference to
In accordance with the present invention, the leads 14 of the lead frame 10 include a coating 16 on at least an upper surface thereof. The coating 16 is made of materials such that the coating 16 has good adhesion with the metal and/or plating of the lead frame, and does not permit solder flux and Tin, which may be inadvertently deposited on the lead fingers 14, to seep through the coating 16 and contact the lead fingers 14. Further, the coating 16 is readily dissolved with a de-coating operation, such as with a flux cleaning solution. In one embodiment, the coating 16 comprises a solution of organic compounds, including about 5.5% cationic surfactants and 93.5% non-hazardous compounds. In one embodiment, the coating is formed of non-hazardous compounds, such as Alkyl-alkene, Aryl-hydrocarbon and relative ramifications, Ester, Oxygenous compounds (organic acid, etc.) and other atomic organic compounds, and cationic surfactants, such as Poly(methyl(γ-(2-hydroxyl-3-N,N-dimethyl-N-benzyl ammonium chloride))-propyl)siloxane.
In order to allow for good bonds to be formed between wires (not shown) and the lead fingers, such as by a wire bonding process, the tin clouds 22 and flux 24 are removed prior to wire bonding.
Referring now to
First, at step 44, the lead frame is dipped into an acid bath in order to remove the oxides The acid may be hydrochloric acid and the lead frame may be dipped bath for about 60 seconds. After the lead frame is removed from the acid bath, at step 46 a first water rinsing operation is performed, preferably with de-ionized water. At step 48, the lead frame is dipped into ammonia in order to neutralize any remaining acid and then at step 50 a second water rinsing operation is performed on the lead frame, again preferably with de-ionized water.
At step 52, the lead fingers of the lead frame are coated with a material that will prevent tin clouding and flux from contaminating the lead frame during a later die attach process. The coating material, as discussed above, comprises a solution of organic compounds, and in one embodiment, the coating comprises about 95% non-hazardous compounds and about 5% cationic surfactants. Then at step 54, a third rinsing operation is performed on the lead frame, again, preferably with de-ionized water.
At step 56, the lead frame(s) are removed from the frame and at step 58, a drying operation is performed. The lead frames may be dried by nitrogen gas at 75 degrees. After drying, at step 60, a visual inspection is performed to check for if there are any defects. The visual inspection step may be performed by machine or operator. Finally, at step 62, the lead frame is packed until such time as it is used in the chip packaging process.
As is evident from the foregoing discussion, the present invention provides a coated lead frame and a method of preparing a coated lead frame that is especially useful for packaging high power circuits. In one embodiment, the coated lead frame is used for forming a power quad flat no-lead (PQFN) packaged device. The present invention allows for improved wire to lead finger bonds and therefore, improved overall package reliability.
The description of the preferred embodiments of the present invention have been presented for purposes of illustration and description, but are not intended to be exhaustive or to limit the invention to the forms disclosed. It will be appreciated by those skilled in the art that changes could be made to the embodiments described above without departing from the broad inventive concept thereof. It is understood, therefore, that this invention is not limited to the particular embodiments disclosed, but covers modifications within the spirit and scope of the present invention as defined by the appended claims.
Li, Zhe, Wang, Chao, Wang, Zhijie, Ye, Dehong, He, Qing Chun
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